It's a boy's way of challenging another boy to a fight. Figuratively, between between nations, it means a limit, beyond which incursion will be treated as an act of war. A literal example of its use between children is this passage, from "Tom Sawyer" by Mark Twain. I've never personally seen this happen in real life. Tom drew a line in the dust with his big toe, and said: "I dare you to step over that, and I'll lick you till you can't stand up. Anybody that'll take a dare will steal sheep." The new boy stepped over promptly, and said: "Now you said you'd do it, now let's see you do it." "Don't you crowd me now; you better look out." "Well, you said you'd do it—why don't you do it?" "By jingo! for two cents I will do it." The new boy took two broad coppers out of his pocket and held them out with derision. Tom struck them to the ground. In an instant both boys were rolling and tumbling in the dirt, gripped together like cats; and for the space of a minute they tugged and tore at each other's hair and clothes, punched and scratched each other's nose, and covered themselves with dust and glory. Presently the confusion took form, and through the fog of battle Tom appeared, seated astride the new boy, and pounding him with his fists. "Holler 'nuff!" said he. The boy only struggled to free himself. He was crying—mainly from rage. "Holler 'nuff!"—and the pounding went on. At last the stranger got out a smothered "'Nuff!" and Tom let him up and said: "Now that'll learn you. Better look out who you're fooling with next time."